The present invention relates to a method of producing a high permeability grain oriented electrical steel from a hot processed strip, or band, comprising about 2.0 to about 4.5% silicon, about 0.1 to about 1.2% chromium, at least about 0.01% carbon and about 0.01 to about 0.05% aluminum. The steel strip will typically have a volume resistivity of at least 45 μΩ-cm, an austenite volume fraction (γ1150° C.) of at least about 20% and an isomorphic layer thickness of at least about 2% of the total thickness of the strip on at least one surface prior to final cold rolling.
Electrical steels are broadly characterized into two classes. Non-oriented electrical steels are engineered to provide uniform magnetic properties in all directions. These steels are comprised of iron, silicon and aluminum to impart higher volume resistivity to the steel sheet and thereby lower the core loss. Non-oriented electrical steels may also contain manganese, phosphorus and other elements commonly known in the art to provide higher volume resistivity and lower core losses created during magnetization.
Grain oriented electrical steels are engineered to provide high volume resistivity with highly directional magnetic properties owing to the development of a preferential grain orientation. These steels are differentiated by the grain growth inhibitors used, the process routing employed and the quality of the grain orientation achieved as indicated by the magnetic permeability measured at 796 A/m. Regular (or conventional) grain oriented electrical steels have a permeability of at least 1780 whereas high permeability grain oriented electrical steels have a permeability of at least about 1840 and typically greater than 1880. Typically, the volume resistivity of commercially produced grain oriented electrical steels range from 45-55 μΩ-cm which is provided by the addition of from 2.95% to 3.45% silicon with iron and other impurities incidental to the method of steelmaking. The processing steps of major importance may include melting, slab or strip casting, slab reheating, hot rolling, annealing and cold rolling.
To achieve the desired magnetic properties in a grain oriented electrical steel, a cube-on-edge grain orientation is developed in the final high temperature anneal of the steel by a process commonly referred to in the art as secondary grain growth. Secondary grain growth is a process by which small cube-on-edge oriented grains preferentially grow to consume grains of other orientations. Vigorous secondary grain growth is primarily dependent on two factors. First, the grain structure and crystalline texture of the steel, particularly the surface and near-surface layers of the steel surface, must provide conditions appropriate for secondary grain growth. Second, a grain growth inhibitor dispersion, such as aluminum nitride, manganese sulfide, manganese selenide or the like, capable of restraining primary grain growth must be provided to restrain primary grain growth until secondary grain growth is complete.
The composition and processing of the steel influence the morphology of the grain growth inhibitor, microstructure and crystalline texture. The typical methods for the production of high permeability grain oriented electrical steels rely on aluminum nitride precipitates or aluminum nitride precipitates in combination with manganese sulfides, and/or manganese selenides for primary grain growth inhibition. Other precipitates may be included in combination with aluminum nitrides, such as copper and the like. The characteristics of the surface and near-surface layers of the steel surface in the hot processed band are important to the development of a high permeability grain oriented electrical steel. This surface region, depleted of carbon and substantially free of austenite and its decomposition products provides a substantially single phase, or isomorphic, ferritic microstructure, and is referred to in the art as the surface decarburized layer. Alternatively, it may be defined as the boundary between the isomorphic surface layers and the polymorphic (mixed phases of ferrite and austenite or its decomposition products) interior layer, such as shear band and the like. Cube-on-edge secondary grain nuclei with the highest likelihood of sustaining vigorous growth and producing a high degree of cube-on-edge grain orientation are contained within the isomorphic layer or, alternatively, near the boundary between the isomorphic surface layers and polymorphic interior layer.
In the development of grain oriented electrical steels with lower core loss, higher volume resistivity steels have been explored. Typically, higher silicon levels are used which require higher levels of austenite-forming elements to maintain a proper proportion, or phase balance, between the austenite and ferrite phases. Carbon is the most common addition to increase the level of austenite.
The use of higher levels of silicon and carbon for the production of high permeability grain oriented steels has caused many manufacturing problems, increasing both the difficulty and cost of production. Higher levels of silicon and carbon lower the solidus temperature which has an important influence on the formation of defects which may occur during high temperature processing such as solidification, slab or strip casting, slab or strip reheating and/or hot rolling. The use of higher levels of silicon and, to a lesser degree, carbon, have reduced physical ductility and increased brittleness, making the steel more difficult and costly to process. Higher levels of silicon, and to a lesser extent, carbon, contribute to less stable secondary grain growth. As the level of silicon increases, the thermodynamic activity of nitrogen increases and the solubility product of the aluminum nitride grain growth inhibitor is reduced. Higher solutionizing temperatures are then required which make processes such as hot band annealing less productive and more costly. Higher levels of carbon, and silicon increase the time required for carbon removal, making decarburization annealing more difficult and costly.
Given the above mentioned circumstances, there has remained a need for an improved method for the production of high permeability grain oriented electrical steels having high volume resistivity and improved processing characteristics. In the method of the present invention, the proper proportions of silicon, chromium and carbon are provided for vigorous and stable secondary grain growth and excellent magnetic quality. The method of the present invention also improves the decarburization process.